J. Serb. Chem. Soc. 72 (2) 151–157 (2007) JSCS–3545
UDC 539.193+546.11:536.722:543.2 Original scientific paper
Evaluation of the intramolecular hydrogen bond enthalpy by means of specialized quantum chemical methods ALEXEI N. PANKRATOV* and ALEXEI V. SHALABAY Department of Chemistry, N. G. Chernyshevskii Saratov State University, 83 Astrakhanaskaya Street, Saratov 410012, Russia (e-mail:
[email protected]) (Received 17 Febryary, revised 14 April 2006) Abstract: By means of the MNDO/H, I-MNDO, MNDO/HB and MNDO/M methods, the values of the enthalpy of intramolecular hydrogen bond (IHB) formation for series of substances have been computed. The applicability of these methods for a semi-quantitative estimation of the IHB enthalpy in compounds with a planar quasicycle has been shown. Keywords: intramolecular hydrogen bond, enthalpy, quantum chemical evaluation, organic compounds with a planar quasicycle. INTRODUCTION
An extensive number of physicochemical properties of chemical compounds, as well as their reactivity, analytical properties, biological activity depend on the presence of intramolecular hydrogen bonds (IHB) and their thermodynamic parameters.1–10 For the directive synthesis of compounds, the quantum chemical evaluation of the IHB energy (enthalpy) is of great interest. Appropriate models for studying IHB are organic molecules with a planar quasicycle. They are widely distributed and play a very important role in chemistry,1,4,5,8,9,11 being promising building blocks for the construction of supramolecular structures with specific receptor properties. For planar quasicyclic systems, the effect of IHB on thermodynamics can be isolated in an explicit form. When evaluating the IHB enthalpy, it would be correct to use the difference in the enthalpy of formation of a conformer with an IHB and that of a rotamer without an IHB, with all other specific interactions being the same as those in the molecule with an IHB (dihedral angle comprising the X–H bond, which takes part in the X–H···Y IHB, changes from 0 to 180°).12,13 For the initial selection of the compounds – "candidates" – to a directive synthesis, with subsequent refinement by means of the ab initio (non-empirical) or DFT (Density Functional Theory) methods, is seems promising to use a simplified preliminary semi-quantitative estimation of the IHB enthalpy on the basis of spe*
Author for correspondence.
doi: 10.2298/JSC0702151P
151
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PANKRATOV and SHALABAY
cially parameterized semi-empirical methods from the MNDO (Modified Neglect of Differential Overlap) group. The above group includes the methods MNDO/H,14,15 I-MNDO,16 MNDO/HB17,18 and MNDO/M,19 which have been shown to be quite adequate in describing the thermodynamics of intermolecular hydrogen bonds.14–20 The examples of their use for studying IHB are of an exclusive character.21 The aim of the present work consisted in establishing the possibility of the evaluation of the IHB formation enthalpy in organic compounds with a planar quasicycle by means of the MNDO/H, I-MNDO, MNDO/HB and MNDO/M methods. RESULTS AND DISCUSSION
The correctness of the use of the above quantum chemical methods was confirmed by reproducing the experimental values of standard gaseous-phase heats of formation of the substances (Table I). It is interesting to note that the heat of formation of salicylic aldehyde computed by all the methods is in agreement with the experimental value for the liquid state. Obviously, in liquid salicylic aldehyde, the intermolecular interactions are relatively weak. TABLE I. Standard gaseous-phase heats of formation (DHf) of the compounds; to obtain the values in kJ mol-1 multiply by 4.184 2-Hydroxybenzoic (salicylic) aldehyde Method of computation DHf/kcal mol–1 MNDO/H –63.19 I-MNDO –64.63 MNDO/HB –61.93 MNDO/M –64.86 Experiment22* –67.2 *Liquid state 2-Hydroxy-2,4,6-cycloheptatriene-1-one (tropolone) Method of computation MNDO/H I-MNDO MNDO/HB MNDO/M Experiment23
DHf/kcal mol–1 –43.07 –43.34 –43.80 –46.03 –37.23
2-Methoxyphenol (guaiacol) Method of computation MNDO/H I-MNDO MNDO/HB MNDO/M Experiment22
DHf/kcal mol–1 –68.29 –68.34 –68.64 –71.36 –65.38
HYDROGEN BOND ENTHALPY
153
Fig. 1. Mulliken charges on the atoms in the tropolone molecule.
The adequacy of the said methods with respect to the problem to be solved also follows from the correlation between the regioselectivity of electrophilic substitution reactions in a series of tropolones and the electron density distribution (computed using methods of the same group) in the molecule of the origin substance of this series. The charges on the atom (Fig. 1) conform with the occurence of the reactions of azo coupling, nitrosation, nitration, sulphonation, halogenation, hydroxylation, hydroxymethylation of tropolones with the involvement of the positions 3, 5 and 7 of the cycle.24–26 The values of the IHB enthalpies computed by means of the programs from the MNDO 90 package20 are presented in Table II (IR means infrared spectroscopy, GLC is gas–liquid chromatography). As can be seen, the aforesaid methods in general reproduce quite well the enthalpies of IHB formation in different compounds. For thiomalonaldehyde and 2-hydroxybenzenethiol, the "experiment", which data are referred to when comparing the results of semi-empirical evaluations, means the computations using the DFT method in the B3LYP version (the papers33,34 report on the computing procedure in detail and substantiate the assurance of the computitions of the IHB enthalpy by the DFT methods). In the course of the optimization of the geometry of the tropolone molecule it was possible to avoid the "falling" of the system into an alternative energy minimum, which corresponds to a non-planar conformation and is an artefact of the MNDO-group methods. In analytical chemistry, the procedure of the relativation of the systematic errors is commonly used when the analytical determination is performed in respect to a somewhat other object, and the result of the analysis is evaluated by a difference, thus the systematic errors of measurement are mutually substracted.35 Obviously, the above procedure is universal, i.e., is applicable to different kinds of measure-
S–H···O S–H···O
8-Hydroxyquinoline
Thiomalonaldehyde**
2-Hydroxybenzenthiol
9.99 4.28 2.84 4.51 2.30 5.62 7.64 6.32
GLC30 GLC31 IR,CCl427 27
IR,CCl427 27
IR,CCl427 IR,CCl432
on paper
B3LYP/6-311++G(d,p) ****Chromatography
B3LYP/6-311++G(d,p)
1.805***
IR,CCl4
3.29
3.45
7.88
****28,29
IR,CCl4
9.37
MNDO/H
IR,CCl427
2.736***
6
8.19
8.45
3.11
3.07
2.98
4.7
8.00
6.25
9.26
–DHexper Experimental method, kcal mol-1 conditions, literature
computations;
O–H···O
3-Methyltropolone
In the thioenolic form;
O–H···O
2-Hydroxy- 2,4,6-cycloheptatriene-1-one (tropolone)
In the enolic form;
O–H···O
2-Benzyl-3-hydroxy-1,4-napthoquinone
***Our
O–H···O
2-Methyl-3-hydroxy-1,4-napththoquinone
**
O–H···O
2-Hydroxy-1,4-naphthoquinone
*
O–H···O O–H···O
1,2-Dihydroxybenzene (pyrocatechol)
O–H···O O–H···O
2-Nitrophenol
O–H···O
Acetylacetone*
4-Methoxy-2-hydroxybenzaldehyde
Hydrogen bond
Compound
3.32
4.62
6.34
8.44
5.89
2.31
4.58
2.87
4.30
11.4
8.79
10.8
6.15
4.42
4.06
7.08
6.35
2.06
4.94
3.27
3.33
7.85
7.29
7.82
3.25
3.26
3.15
10.3
8.59
5.49
7.62
6.01
7.38
10.8
10.2
10.9
I-MNDO MNDO/HB MNDO/M
–DHtheor/kcal mol-1
TABLE II. Experimental and computed values of the enthalpies of formation (DH) of intramolecular hydrogen bonds; to obtain the values in kJ mol-1 multiply by 4.184
155
HYDROGEN BOND ENTHALPY
ments and theoretical estimations, including quantum chemical computations. Relativation of the errors of the differential evaluation of the energetic characteristics of IHB for 2-nitrophenol leads to satisfactory agreement between the calculated and experimental values of the IHB enthalpy, in spite of the poor reproduction of the heats of formation of nitro compounds by the MNDO computing scheme.36–40 TABLE III. Relationship between the experimental and computed values of the enthalpy of intramolecular hydrogen bonds DHexper = bDHtheor Method
b
r
MNDO/H
0.9440±0.1366
0.8685
I-MNDO
0.8494±0.1363
0.8474
MNDO/HB
0.9907±0.2144
0.6871
MNDO/M
0.7333±0.1389
0.7676
The semi-empirical nature of the MNDO-group methods results sometimes in the over estimation or underestimation of the IHB enthalpy. Thus, the MNDO/M method overestimates the IHB enthalpy in the cases of 4-methoxy-2-hydroxybenzaldehyde, 2-hydroxy-1,4-naphthoquinone, 2-methyl-3-hydroxy-1,4-naphthoquinone, and underestimates it for the molecule of 8-hydroxyquinoline. The MNDO/HB method overestimates the IHB enthalpy in the 2-hydroxythiophenol molecule. Table II is in essence a recommendation for choosing a given method from the MNDO group with the purpose of the computation of the IHB enthalpy in compounds from one or other series. When using the MNDO/H and I-MNDO methods, the interrelationship between the computed and experimental values of the IHB enthalpy tends to linearization, although a true linear dependence is not the case in point (Table III; the number of compounds m = 12; r is the correlation coefficient). It can not be rigorously concluded therewith that any method, in general, gives better or worse results as compared to the others. On the one hand, for MNDO/HB, the slope b of the dependence DHexper = bDHtheor is essentially closer to unity. However, on the other hand, the confidence limits concerning the slope are considerably greater in this case than in the other ones, and the correlation coefficient is the lowest among the data of all the methods. Thus, for each series of compounds, only the testing of all the specialized methods considered in this work on a species – selected representative of the sample – as an example would enable a decision to be made between the methods. The studies carried out show the principal applicability of the MNDO/H, I-MNDO, MNDO/HB, MNDO/M methods for a priori semi-quantitative evaluation of the heat of IHB formation in compounds with a planar quasicycle, including preliminary estimations before proceeding to more sophisticated quantum chemical methods and forthcoming planned synthesis.
156
PANKRATOV and SHALABAY
IZVOD
PROCENA ENTALPIJE INTRAMOLEKULSKIH VODONI^NIH VEZA ODRE\ENIM KVANTNOHEMIJSKIM METODAMA ALEXEI N. PANKRATOV i ALEXEI V. SHALABAY Department of Chemistry, N. G. Chernyshevskii Saratov State University, 83, Astrakhanskaya Street, Saratov 410012, Russia
Primenom metoda MNDO/H, I-MNDO, MNDO/HB, MNDO/M su izra~unate vrednosti entapije intramolekulskih vodoni~nih veza serije razli~itih supstanci. Pokazano je da se metode mogu primeniti u odre|ivawu entalpije intramolekulskih vodoni~nih veza planarnih kvazicikli~nih jediwewa. (Primqeno 17. februara, revidirano 14. aprila 2006)
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